There has been significant effort expended in recent research on advanced semiconductor devices which rely upon very thin layers of a particular semiconductor composition. For instance, in one type of quantum well laser, an active layer of GaAs separates two layers of GaAlAs. Although the thickness of the GaAlAs layers need not be precisely controlled, the thickness of the GaAs layer must be precisely controlled to a predetermined number of atomic layers (monolayers) falling within the range of 10 to 100 monolayers. The accuracy of the thickness must be in the range of one to two monolayers.
One method of achieving such control in a III-V semiconductor is atomic layer epitaxy (ALE) as described by Den Baars et al in a technical article entitled "GaAs/AlGaAs quantum well lasers with active regions grown by atomic layer epitaxy" appearing in Applied Physics Letters, Vol. 51, No. 9, 1987 at pages 1530-1532. By atomic layer epitaxy is meant an epitaxial crystal growth process in which a single atomic layer of a crystal, usually a III-V crystal, is deposited on a substrate before the fabrication process is changed and the next atomic layer is grown. In the past, ALE has usually relied on each of the deposition steps being self-limiting so that the exact time devoted to depositing a single layer is not crucial. However, for purposes of this invention, a self-limiting growth process is not assumed.
Den Baars et al used an organometallic chemical vapor deposition (OMCVD) method to be described in more detail later. In OMCVD growth of GaAs, a substrate is exposed to vapors of trimethylgallium (TMGa), which supplies the Ga, and of arsine (AsH.sub.3), which supplies the As. The relatively thick GaAlAs layers were grown by the conventional OMCVD method of exposing the substrate simultaneously to TMGa, trimethylaluminum (TMAL) and AsH.sub.3. However, the intermediate thin GaAs was required to be 6 nm thick, about 20 GaAs monolayers, and was grown by ALE. In ALE, the (001) substrate was alternately exposed to AsH.sub.3 and to TMGa. The exposure times were sufficient to saturate the bonding. Therefore, during the AsH.sub.3 exposure, virtually all Ga atoms exposed at the surface became bonded with As so that a monolayer of As was formed over the underlying layer of Ga. Similarly, the exposure to TMGa produced a surface monolayer of Ga overlying the previously formed As monolayer. The above process is repeated to form a precise number of monolayers.
Den Baars et al used fixed exposure times derived from previously obtained growth rates, which they showed to be very temperature dependent near their ALE temperature of 485.degree. C. It is believed that such control is satisfactory for the As deposition since As bonded to As will sublime at rates higher than typical deposition rates even at temperatures as low as 300.degree. C. Therefore, any excess As, that is, As unbonded to the underlying Ga and perhaps still in the form of AsH.sub.3, immediately evaporates when the supply of AsH.sub.3 is interrupted so that the As is self-limiting. However, Ga does not exhibit such a high vapor pressure (or low sublimation temperature). Therefore, if there has been an excess exposure to TMGa, the Ga condenses into Ga globules on the substrate surface. Upon removal of the TMGa vapor, the Ga in the globules is available for bonding to the As in the subsequently supplied AsH.sub.3. Therefore, during the supply of AsH.sub.3, the globular Ga can contribute to the formation of one or more GaAs layers. Of course, complete coverage for a Ga or an As monolayer is desired. Hence, for precise monolayer thickness control, exposure times must be accurately controlled in the presence of possible process variations such as temperature and pressure.
It would be desirable to have a real-time or in situ monitor for determining when a monolayer has in fact been deposited and to thereby control the supply of the vapor phases. One well known in situ method for surface analysis is reflection high-energy electron diffraction (RHEED). In RHEED, a high energy electron beam of approximately 8 keV is directed to the substrate surface at a fixed angle and an electron beam thereby reflected or diffracted from the surface at a corresponding angle is monitored. RHEED tests the surface structure. During monolayer formation, the RHEED signal changes, notably periodically in synchronism with the formation of multiple monolayers.
RHEED, however, suffers many drawbacks. RHEED tests the surface structure rather than the chemistry producing that structure. The high energy electrons are destructive and must be directed to a sacrificial portion of the substrate. Most importantly, RHEED is operable only in ultra-high vacuums. As such, RHEED is usable with molecular beam epitaxy (MBE) but cannot be used with OMCVD, which operates near atmospheric pressure. Other types of vapor phase crystal growth also operate at pressures higher than would permit the use of RHEED.